1. Field of the Invention
The present invention relates generally to a wireless power transmitter and method of controlling the same, and more particularly, to a wireless power transmitter and a method for performing communication according to an adjusted transmission power.
2. Description of the Related Art
Mobile terminals, such as cell phones, Personal Digital Assistants (PDAs), etc., are powered by rechargeable batteries, and in order to recharge the batteries, the terminals supply electric energy to the batteries via separate charging devices. Typically, the charging device and the battery each having contacting terminals on their respective outer surfaces, and are electrically connected to each other via their contacting terminals.
However, when using such a contact charging method, the contacting terminals are susceptible to contamination by dirt, due to their exterior location, and such contamination may result in inadequate charging. Also, the rechargeable batteries may not be properly charged when the contacting terminals are exposed to moisture.
To address these problems, wireless charging or contactless charging technologies have recently been developed and applied to many different electronic devices.
A wireless charging technology using wireless power transmission and reception enables, for example, a battery of a cell phone to be automatically charged just by placing the cell phone on a charging pad without a need of a separate charging connector. Such technology is currently applied to wireless electric toothbrushes or wireless electric shaver. From the wireless charging technology, the electronic device may be benefited with enhanced waterproof and portable functions because of no need for a wired charging device. And in the coming era of electric vehicles, various relevant technologies are expected to be even more developed.
The wireless charging technology has an electromagnetic induction method using coils, a resonance method using resonance, and a Radio Frequency (RF)/micro wave radiation method that converts electric energy into microwaves for transmission.
Although wireless charging technology has thus far been dominated by the electromagnetic induction method, due to recent successful experiments in microwave-based wireless transmission from distances of a few tens of meters between devices, it is foreseeable that, in the near future, all electronic products may be wirelessly recharged anywhere and anytime.
A power transmission method based on the electromagnetic induction transfers power between primary and secondary coils. Movement of a magnet through a coil produces an induced current based on which a magnetic field is produced at the transmission end, and the change in the magnetic field at a receiving end induces a current to generate energy. This phenomenon is referred to as magnetic induction, and power transmission methods based on the magnetic induction provide superior energy transmission efficiency.
In a resonance method for wireless charging, a professor Soljacic of the Massachusetts Institute of Technology (MIT) suggested a system in which electricity is delivered wirelessly, even when the system is a few meters away from a charging device, using a resonance-based power transmission principle based on Coupled Mode Theory. The MIT team's wireless charging system is based on the resonance effect, a physical concept where a tuning fork being placed next to a wine glass causes the wine glass to ring with the same frequency. In the resonance method electromagnetic waves carrying the electric energy are resonated instead of sound. Resonant electric energy of electromagnetic waves is directly transferred only when there is a device having the same resonant frequency, and the non-used part of the energy is re-absorbed into the magnetic field rather than being dispersed in the air, and thus the resonant electric energy has not been found to be harmful to surrounding machines or bodies.
Studies on wireless charging methods are actively being performed, but standards for prioritizing wireless charging, searching for wireless power transmitters/receivers, selecting the communication frequency between the wireless power transmitter and receiver, adjusting the wireless power, selecting a matching circuit, distributing communication time for each wireless power receiver in a single charging cycle, etc. have not been suggested. In particular, there is a need for standardization for configurations related to wireless charging, and a need for procedures for providing an appropriate power for each wireless power transmission by wireless power transmitters.